When a video signal having M levels of tone is input into a display device that can display a video signal having up to N levels of tone (N<M) (M and N are natural numbers), the display device cannot display (express) all information of the M tone levels of the input signal. In that case, the display device, such as a plasma display device, uses a technique for expressing a video image corresponding to the input video signal as faithfully as possible only using the tone levels that can be displayed with the device. Error diffusion is one such technique (one such process).
Error diffusion is to distribute (diffuse) an error that is generated through tone level restriction at a pixel of an I-th frame (I is a natural number) or at a pixel of a frame preceding the I-th frame to other pixels (unprocessed pixels) of the I-th frame at which tone level restriction is yet to be performed and to pixels of an (I+1)th frame or frames following the (I+1)th frame. This technique enables the tone levels that cannot be displayed with the display device to be expressed using a plurality of other pixels in a spatial direction (pixels within the same frame) and a plurality of other pixels in a temporal direction (pixels at the same position in different frames and their neighboring pixels). This technique enables the display device to produce a video image with good reproducibility of tone levels.
However, one problem associated with this technique is that flicker may occur when an error generated in the I-th frame is distributed to the (I+1)th and following frames. Flicker occurs when the error accumulates through repeated distribution, and can cause pixels of one frame to have values different from the values of the corresponding pixels of the preceding and following frames. For ease of explanation, error diffusion is assumed to be performed only in the temporal direction. In this case, as shown in FIG. 2, the pixels of the I-th to the (I+3)th frames at the same position each have a pixel value of 40, whereas the corresponding pixel of the (I+4)th frame has a pixel value of 50. Such pixel value deviation causes flicker to occur in a video image displayed by the display device.
To overcome this problem, one method proposes to reduce unnecessary noise and reduce flicker by calculating the absolute value of a difference between a current video signal (a target pixel) and a video signal (pixel) delayed in each of a horizontal direction, a vertical direction, and a temporal direction of an image formed using the current video signal, determining that a smaller absolute value of the difference between the signals means that the two signals have a higher correlation, and distributing an error of the target pixel at a higher rate to a pixel determined to have a higher correlation with the target pixel (see, for example, Patent Citation 1).
FIG. 17 shows the conventional technique (conventional image processing device (error diffusion device)) described in Patent Citation 1.
A dot storage unit 102, a line storage unit 103, and a frame storage unit 1401 shown in FIG. 17 delay an input signal (input video signal), and calculate a difference between a target pixel and its corresponding pixel in each of a horizontal direction, a vertical direction, and a frame direction (temporal direction). Although the term “field” refers to a group of video signals in Patent Citation 1, the components in FIG. 17 use the term “frame” to replace the field because whether to use a frame or a field is not essential to the technique.
Absolute value calculation units 1409A to 1409C each calculate the absolute value of an input difference, and output the calculated absolute value to a weight determination unit 1404.
The weight determination unit 1404 receives the absolute values of the differences output from the absolute value calculation units 1409A to 1409C, and calculates a weighting coefficient of each pixel in a manner that the error generated at the target pixel is distributed at a higher rate to a pixel having a smaller absolute value of the difference from the target pixel.
An error addition unit 105 adds an error to the input signal, which is delayed to adjust its processing timing, and outputs the resulting signal to a tone level restriction unit 1406.
The tone level restriction unit 1406 outputs, to a dot error storage unit 14071, a line error storage unit 14072, and a frame error storage unit 1408, the upper n bits of the input signal to which the error has been added ((m+n)-bit signal) as an output signal (output video signal) and the lower m bits of the signal as an error element.
Each of the dot error storage unit 14071, the line error storage unit 14072, and the frame error storage unit 1408 receives the error element output from the tone level restriction unit 1406. Each of the dot error storage unit 14071, the line error storage unit 14072, and the frame error storage unit 1408 first delays the error element and then multiplies the error element by its weighting coefficient to generate a weighted error element, and outputs the weighted error element to the error addition unit 105.    Patent Citation 1: Japanese Unexamined Patent Publication No. 2000-155565